Difference MySQL PostGreSQL
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Contents
Introduction
The biggest differences are:
- PostGreSQL is case sensitive
- Command/function
- Dates are stored without format in PostGreSQL
- Round off
- Customised functions
Changes in database views from MySQL to PostreSQL
When upgrading from IMSMANG 5.08.04 to 6.0, the database views need to be adapted, i.e. changed from MySQL syntax to PostgreSQL syntax. The country focal point is taking care of this. For reference, this section describes the main changes encountered. This can also be used as a tutorial for creating views in PostgreSQL, for IMSMANG administrators who were used to creating them in MySQL.
Case sensitivity and column names
PostgreSQL, as opposed to MySQL, is case-sensitive. This means that strings need to be provided in exactly the way they are stored in the database. For example, in the table IMSMAENUM, one out of many values in the column ENUMVALUE is Progress Report, with capital P and capital R. In MySQL it was possible to write:
SELECT * FROM imsmaenum WHERE enumvalue = 'progress report'
and still get one row as result. In PostgreSQL, the same query will return no result.
Therefore, a create view statement like this:
MySQL
CREATE OR REPLACE VIEW my_view AS SELECT ... FROM imsmaenum, ... WHERE imsmaenum.enumvalue = 'progress report';
needs to be changed to:
PostgreSQL
CREATE OR REPLACE VIEW my_view AS SELECT ... FROM imsmaenum, ... WHERE imsmaenum.enumvalue = 'Progress Report';
Similarly, in MySQL it is possible to define column names with a mix of upper and lower case as well as blanks and other special characters without any trouble. In PostgreSQL, this is also possible, but in this case the column name must be enclosed in double quotes. For example, the following create view statement:
MySQL
CREATE VIEW my_view AS SELECT hazreducdeviceinfo.hazreduc_guid AS hazreduc_guid, ordnance.model AS Device Type FROM hazreducdeviceinfo;
needs to be changed to:
PostgreSQL
CREATE VIEW my_view AS SELECT hazreducdeviceinfo.hazreduc_guid AS hazreduc_guid, ordnance.model AS "Device Type" FROM hazreducdeviceinfo;
This also applies to querying the data. For select rows where the device type is equal to 'xy', the syntax is the following: ... WHERE "Device Type" = 'xy' |
If an column name in PostgreSQL is written with upper case but not enclosed into double quotes, it will be created in lower case in the database. |
CAST AS CHAR CHARSET utf8 and _utf8 (mySQL)
In some (older) MySQL versions, UTF-8 was not the default character set and strings thus needed to be explicitly converted to UTF-8. This is not the case in PostgreSQL and UTF-8 related syntax as used in MySQL does not work in PostgreSQL. Below are two examples encountered in IMSMANG views.
MySQL
AND (`customdefinedfield`.`label` = _utf8'CDF-Area Reduction MDU')
PostgreSQL
AND (customdefinedfield.label = 'CDF-Area Reduction MDU')
MySQL
cast(`geopoint`.`latitude` as char charset utf8) AS `lat`
PostgreSQL
cast(geopoint.latitude as char) AS lat
IF and IFNULL (mySQL) / CASE (PostgreSQL) statements (conditional expressions)
The statements IF() and IFNULL() do not exist in PostgreSQL. In PostgreSQL, the CASE statement is used for conditional expressions. The syntax of the CASE statement is as follows:
CASE WHEN condition THEN result [WHEN ...] [ELSE result] END
Therefore, for example the following create view statement:
MySQL
CREATE VIEW `my_view` AS SELECT sum(if((`my_view2`.`Device Type` = _utf8'SAA'),`my_view2`.`Quantity`,0)) AS`SAA`, sum(if((`my_view2`.`Device Type` = _utf8'AP'),`my_view2`.`Quantity`,0)) AS `AP`, sum(if((`my_view2`.`Device Type` = _utf8'AIED'),`my_view2`.`Quantity`,0)) AS `AIED`, sum(if((`my_view2`.`Device Type` = _utf8'UXO'),`my_view2`.`Quantity`,0)) AS `UXO`, sum(if((`my_view2`.`Device Type` = _utf8'AT'),`my_view2`.`Quantity`,0)) AS `AT`, sum(if((`my_view2`.`Device Type` = _utf8'Fragments'),`my_view2`.`Quantity`,0)) AS `Fragments` FROM `my_view2`;
needs to be changed to:
PostgreSQL
CREATE VIEW my_view AS SELECT sum(case when my_view2.Device Type = 'SAA') then my_view2.Quantity else 0 end) AS "SAA", sum(case when my_view2.Device Type = 'AP') then my_view2.Quantity else 0 end) AS "AP", sum(case when my_view2.Device Type = 'AIED') then my_view2.Quantity else 0 end) AS "AIED", sum(case when my_view2.Device Type = 'UXO') then my_view2.Quantity else 0 end) AS "UXO", sum(case when my_view2.Device Type = 'AT') then my_view2.Quantity else 0 end) AS "AT", sum(case when my_view2.Device Type = 'Fragments') then my_view2.Quantity else 0 end) AS "Fragments" FROM my_view2;
The ISNULL() function can also be translated into a CASE statement:
CASE WHEN field IS NULL THEN result END
Date manipulations
There are differences between MySQL and PostgreSQL when it comes to manipulating fields of type DATE, such as for example extracting the year or the month of a date. In MySQL, the related functions are year() and month(). In PostgreSQL, there is an extract() function that does the job. See the below example for reference. The following create view statement:
MySQL
CREATE VIEW `my_view` AS SELECT (year(`my_view2`.`DateofAccident`) - year(`my_view2`.`DateofBirth`)) AS `Age`, year(`my_view2`.`DateofAccident`) AS `Year`, month(`my_view2`.`DateofAccident`) AS `Month` FROM `my_view2`;
needs to be changed to:
PostgreSQL
CREATE VIEW `my_view` AS SELECT extract(year from my_view2."DateofAcciden") - extract(year from my_view2."DateofBirth") AS "Age", extract(year from my_view2."DateofAccident") AS "Year", extract(month from my_view2."DateofAccident") AS "Month" FROM my_view2;